It is known to employ acoustic energy to manipulate particles suspended in a fluid, for example, to separate different types of particles from a liquid or from each other. Prior known devices and techniques, however, do not adequately meet certain industry requirements for manipulating fluid-borne particles. In particular, there is need for devices and methods having greater functionality in collecting or manipulating fluid-borne particles for testing and analysis of fluid samples, such as chemical process stream samples, environmental fluid samples, e.g., river or lake water to be tested for pollution levels, biological fluids, e.g., blood or other fluids containing cells or other biological particles, etc.
The establishment of a standing wave in a fluid is generally believed to result in the formation of velocity nodes or antinodes to which particles migrate, depending on their compressibility or density, especially their compressibility or density relative to that of the fluid in which the particles are carried or suspended. Most solid and liquid particles (liquid particles here meaning substantially discrete globules, bubbles or other bodies of liquid in a gaseous or liquid fluid having a sufficient difference of compressibility or density to permit manipulation of the particles by ultrasonic standing waves set up in the fluid) move toward the velocity antinodes. Nodes and antinodes (referred to here generically as “nodes” in some cases) are typically at right angles to the direction of propagation of the sound waves through the fluid, and the nodes are spaced from adjacent nodes by a distance equal to one half of the wavelength of the acoustic wave. The aggregating effect of ultrasonic sound within these antinodes has already been described in the literature. From E. Skudrzyk, “Die Grundlagen der Akustic,” Springer Verlag, Wien, 1954, S. 202-205, S. 807-825; L. Bergmann, “Der Ultraschall und seine Anwendungen in Wissenschafi und Technik,” Verlag hirzel, Zuerich, 1954: as well as K. Asai and N. Sasaki, “Treatment of slick by means of ultrasonics,” Proceedings of the 3rd International Congress on Coal Preparation, Institut National de l'Industrie Charbonniere, Brussels-Liege, 1958, all of which are incorporated herein by reference in their entirety for all purposes. The frequency to be used in the applied sound is best chosen within the magnitude of the so-called characteristic frequency f0. Using this frequency range, the effect of radiation force and cumulative acoustically induced Bernoulli forces within the antinode planes can generally be maximized.
According to U.S. Pat. No. 4,055,491, ultrasonic standing waves have been used to flocculate small particles, such as blood or algae cells, within velocity antinodes of an acoustic field, so that they settle out of the carrying liquid by gravity. But the undefined placement of the ultrasonic source and therefore low efficiency of the standing wave field due to undefined resonance boundary conditions result in high energy losses due to a considerable fraction of traveling waves.
Other methods and devices for separating different types of particles, that is, the manipulation of particulate matter in a fluid medium by the use of ultrasonic wave energy, including the segregation of dissimilar particles from a mixture of particles, are described in U.S. Pat. Nos. 4,673,512 and 4,877,516 to Schram. An ultrasonic standing wave in Schram is propagated in a liquid medium, with relative motion between the liquid and the standing wave. The different types of particles in the liquid are said to be differently influenced by the acoustic energy of the standing wave and/or the Stokes or drag forces of the liquid. The different particle types are said to move at different rates with respect to the standing wave and to be thereby progressively separated by cyclically varying the acoustic energy propagation. U.S. Pat. No. 4,673,512 introduces an interference standing wave field generated by opposing transducers which are excited with the same frequency. By controlling the phase shift between the electric excitation signals of the two acoustic sources, it is said to be possible to move-particles trapped within the antinodes or nodes of the traveling interference pattern in the dispersion. Gravity forces are said in U.S. Pat. No. 4,877,516 to cause some degree of vertical separation of different particle types, i.e., each type of particle will be located at a particular height representing an equipotential plane, dependent upon the influence of gravity on that particle type. The U.S. Pat. No. 4,887,516 suggests controlled movement of local gradients of the acoustic amplitude of the standing field perpendicular to the direction of sound propagation. Thus, particles are moved within the antinodes or nodes of the field by the Bernoulli-force which is directly related to described gradients and is acting parallel to the anti-node planes. A disadvantage of this arrangement is the requirement of mechanically moving an array to produce acoustic shadows in order to achieve desired movement of local gradients of the standing wave. Stepwise movement of the antinodes of a resonant standing wave by exciting succeeding resonance modes of a resonator system is described in PCT patent application No. PCT/AT89/00098. Although resonance boundary conditions are fulfilled in some of the described embodiments, there would appear to be considerable acoustically induced dissipation due to the resonator frequencies used, which are close to an Eigen-frequency of the transducer.
Particle separation is said to occur also in the apparatus of U.S. Pat. No. 4,523,632 to Barmetz et al. Particles of different types are said to be separated to some degree as they are carried by a liquid flow along the length of a horizontal chamber in which a standing wave is established with a wavelength that is half the height of the chamber.
In U.S. Pat. No. 4,879,011 to Schram particulate material is said to be supported in a fluid medium by means of an ultrasonic standing wave while a reaction is effected or controlled involving the particulate material, for example, reaction with the fluid medium or other material contained in the medium. Schram suggests that the standing wave can be established by opposed ultrasonic transducers producing convergent beams that compensate for attenuation of the ultrasonic energy in the fluid medium, and operate in the near field. The support provided by the standing wave is said to be able to avoid settling of the particulate material in the medium and to agitate the material. Both effects are said to enhance the rate of chemical reaction and help to ensure that the particulate material is more uniformly exposed. The process is said to have application to biological reactions, such as fermentation, and to chromatography, etc.
The application of ultrasonic standing waves has also been suggested for separating particles with various acoustic qualities in U.S. Pat. No. 4,280,823 and U.S. Pat. No. 4,523,682. Specifically, these patents suggest reliance on the differential effect of acoustic forces on particles having differences in density, speed of sound or size. Thus, a device to separate particles with various acoustic qualities is described in U.S. Pat. No. 4,523,682, wherein a low resonance mode of a vessel containing a dispersion is excited by a relatively small transducer mounted at one end of the vessel, resulting in node and antinode planes perpendicular to the transducer/vessel interface. A disadvantage of such prior methods and devices, however, lies in the fact that a further non-acoustic force—such as gravity or frictional force—is necessary in order to achieve a successful separation. Furthermore, both patents rely on having permanently and consistently constant acoustic environments. The above methods generate microscopic areas of increased particle concentration (areas of antinodes and nodes), but they only truly generate a macroscopical separation, i.e., one that extends across several wave lengths, by means of a non-acoustic force, primarily gravity.
In U.S. Pat. Nos. 5,527,460 and 5,626,767 to Trampler et al., particulate material in a fluid suspension is said to be separated and recycled by means of an ultrasonic resonance wave or field generated within a multilayered composite resonator system. A transducer, the suspension and a mirror are all said to be parallel to each other. Dimensions and frequencies resonant to the whole system but not exciting Eigen-frequencies of the transducer and mirror are said to be chosen in order to minimize thermal dissipation. Specialized applications in biotechnology are described, including an acoustic filter for mammalian cell bioreactors or the selective retention of viable cells relative to non-viable cells. The systems of Trampler et al, however, are gravity dependent to the extent they use acoustically induced forces to retain and aggregate dispersed particles and use gravity to settle and recycle the aggregates.
As noted above, there is a significant need for improved devices and methods for handling fluid-borne particles, such as fluid test samples, biological fluids and other fluids comprising fluid-borne particles. It is an object of the present invention to address the need for such improved devices and methods. In particular, it is an object of at least certain preferred embodiments of the invention to provide improved devices and methods with greater functionality in collecting and manipulating fluid-borne particles for testing and analysis of fluid samples, such as chemical processing stream samples, environmental fluid samples, e.g., river or lake water to be tested for pollution levels, and biological fluids, e.g., blood or other fluids containing cells or other biological particles, etc. These and other objects and features of the methods and devices of the present invention will be better understood from the following disclosure and detailed description of certain preferred embodiments.